Various integrated circuit devices can be formed in a semiconductor chip which includes an epitaxial layer comprising at least two portions of different thicknesses. Such an epitaxial layer facilitates the inclusion in the integrated circuit of circuit elements, such as transistors, having different properties. For example, transistors that have relatively small geometries and relatively high switching speeds are localized in an epitaxial portion of narrow thickness. These small geometry transistors tend to be vulnerable to surges of high voltage and/or high current. To protect the small geometry (size) transistors against surges of high current and/or high voltage, transistors of larger geometry (size), but slower switching speed, are localized in an epitaxial portion of larger thickness. Additionally, a common semiconductor chip having epitaxial portions of different thicknesses may facilitate the inclusion of diverse circuit elements, such as diodes, resistors, capacitors and transistors. Still further, epitaxial portions of different thicknesses may be used to facilitate the fabrication in a common chip of Complementary Metal-Oxide-Semiconductor (CMOS) circuits and/or circuits involving combinations of bipolar and insulated gate field effect transistors.
For these reasons the prior art includes integrated circuit devices using a semiconductor chip including epitaxial layers having portions of different thicknesses. Various techniques have been used hitherto to prepare such a chip.
One process involves forming on a monocrystalline semiconductive substrate an epitaxial layer of a desired first thickness and then, after masking the substrate, selectively thinning some portions of the layer to a desired smaller thickness.
Another process includes: forming two cavities of different depths in a suitable substrate; growing an epitaxial layer of a first thickness that completely fills the shallower cavity but only partially fills the deeper cavity; and then, after masking the region of the shallower cavity, completing the filling of the deeper cavity with a second localized epitaxial deposition.
Still another process involves: forming a first epitaxial layer over a suitable substrate in a deposition chamber; removing the substrate from the deposition chamber to thin or remove selected portions of this epitaxial layer; and then returning the substrate to the deposition chamber for resuming the growth of epitaxial material over the substrate.
In general, the need for plural insertions and withdrawals of the substrate into and out of a deposition chamber adds to the complexity of a process for forming an epitaxial layer having portions of different thicknesses. Typically the greater the complexity of a process, the higher its cost and the lower its yield.